1. Field of the Invention
The present invention concerns a method in order to generate and correspondingly present diffusion information (diffusion-weighted MR images, for example) by operation of a magnetic resonance system, as well as a correspondingly designed magnetic resonance system.
2. Description of the Prior Art
In routine clinical practice, diffusion-weighted magnetic resonance (MR) images supply important diagnostic information in stroke and tumor diagnostics. In diffusion-weighted imaging (DWI), diffusion gradients are switched (activated) in specific directions, and the diffusion of water molecules along the applied diffusion gradients attenuates the measured magnetic resonance signal. In areas with lower diffusion, a lower signal attenuation thus takes place, so that these areas are imaged with higher image intensity in an imaging magnetic resonance tomography (MRT) measurement (data acquisition). The strength of the diffusion weighting is thereby correlated with the strength of the applied diffusion gradient. The diffusion weighting can be characterized with an attribute known as the b-value, which is a function of gradient parameters (for example the gradient strength, the duration with which the diffusion gradients are applied, or the interval between the applied diffusion gradients).
In diffusion-weighted MR measurements, there is also the possibility to examine the orientation dependencies of the diffusion. In this form of diffusion imaging, multiple MR images (30 to multiple 100s) are normally acquired with different diffusion directions (directions of the diffusion gradients) and combined with one another in order to calculate parameter maps (ADC (“Apparent Diffusion Coefficient”) or FA (“Fractional Anisotropy”)), for example. These data are also used in “Diffusion Tractography” studies, for example in order to obtain information about the anatomy of nerve tracts. The time duration to acquire such diffusion-weighted MR image series is normally between 5 and 20 min, but can also be longer. Since the diffusion-weighted MR images are generated in chronological order during this time period, the numerous diffusion-weighted MR images must subsequently be brought into registration in order to take a movement of the subject into account. The registration of two or more MR images means the implementation of an algorithm or procedure in which MR images that depict the same subject are brought into congruence with one another in the best possible manner. This registration proceeds in two parts: the first part is a motion detection in order to determine a spatial transformation that describes the relative positions and orientations of the two images; the second part is a “regridding”, wherein the spatial transformation is used in order to bring the position and orientation of the subject in the two images into agreement. An incorrect consideration of a subject movement leads to a flawed calculation of the diffusion results or diffusion information.
In U.S. Pat. No. 5,539,310 subject movement is detected by means of a cost function that depends only slightly on contrast changes of the MR images. Such a method shows poor results in the registration of MR images specifically at high b-values, since the image contrast has a strong dependence on the direction of the diffusion gradients.
The article “Comprehensive Approach for Correction of Motion and Distortion in Diffusion-Weighted MRI”, G. K. Rhode et al., Magnetic Resonance in Medicine 51: 103-114, (2004) also describes a cost function that is based on the so-called transinformation (“mutual information”) of the MR images to be registered. This cost function is used in order to register the diffusion-weighted MR images.
The article “Spatial Registration and Normalization of Images”, K. J. Friston et al., Human Brain Mapping 2:165-189, (1995) discloses an error-squared cost function that is used in functional magnetic resonance imaging, for example.
According to “Characterization of Anisotropy in High Angular Resolution Diffusion-Weighted MRI”, L. R. Frank et al., Magnetic Resonance in Medicine 47: 1083-1099, from 2002, a series of first MR images acquired with a b-value=0 is interleaved with an acquisition of second MR images with a high b-value. The first MR images are brought into registration with one another, and the parameters obtained from this registration are used in the registration of the second MR images. This method has a low temporal resolution since in general only a few first images (with a b-value=0) are acquired, such that the time interval between two successive first images is relatively large.